This invention relates to a method for measuring the thickness of a film for measuring the thickness of a sheet such as magnetic tape.
FIG. 1 is a schematical diagram showing a conventional film thickness measuring apparatus disclosed in published Japanese Patent Application No. 60-174609 for example. In the figure, reference numeral 1 is a rotary shaft rotated at a predetermined speed, 2 is a light shield disposed parallel to the rotary shaft 1 at a predetermined distance from the surface of the rotary shaft 1, 3 is a sheet to be measured which is in intimate contact with the rotary shaft 1 and fed at the same speed as the rotary shaft 1, 4 and 5 are laser oscillators disposed at predetermined angles for generating laser beams 4a and 5a, respectively, 6 is a reflective mirror which is controlled to scan between the surface of the rotary shaft 1 and the light shield 2 when it receives the laser beam 4a and which is controlled to scan between the sheet 3 and the light shield 2 when it receives the laser beam 5a. 7 and 8 are lenses for collecting the laser beams 4a and 5a reflected from the reflective mirror 6, 9 and 10 are lenses for collecting the laser beam 4[a and 5a, 11 and 12 are light receivers, 13 and 14 are counters, 15 is an operator and 16 is a display.
The operation will now be described. The laser beams 4a and 5a emitted from the laser oscillators 4 and 5 are reflected at the reflective mirror 6 to scan at an equal angular velocity. The reflected laser beams 4a and 5a are focused at the lenses 7 and 8 to have the smallest beam diameters at the gap positions A (between light shield 2 and shaft 1) and B (between light shield 2 and film 3) of the gap as shown in FIGS. 1 and 2 and are scanned at a constant speed in a direction perpendicular to the rotary shaft 1 or in the direction of the gap. At this time, the light receivers 11 and 12 receive laser beams only while the laser beams 4a and 5a are traversing the gaps A and B.
Therefore, the output signals from the light receivers 11 and 12 are pulses having pulse widths proportional to the widths of the gaps A and B. These pulses are counted by the counters 13 and 14 to obtain count numbers corresponding to the pulse widths. The calculator 15 calculates the thickness from the count numbers and displays it on the display 16. When the count number of the counter 13 is a and the count number of the counter 14 is b, the thickness t.sub.x of the sheet 3 to be measured can be obtained by the following equation (1): EQU t.sub.x =t.sub.0 (1-b/a ) (1)
where, t.sub.0 is the width (dimension) of the gap A.
Since the conventional method for measuring the film thickness is constructed as above described, the film thickness distribution in the width direction of the sheet cannot be measured.